This invention relates to a thermomagnetic recording and reproducing apparatus, particularly for recording and reproducing data on a magnetic medium such as a tape by heating the medium along a continuous path with a focused laser beam.
Various types of thermomagnetic recording and reproducing devices have been proposed. For example, such a device may use a recording medium composed of a thin film of MnBi and utilize the temperature dependency of the coercive force of such a thin film by focusing a laser light beam along a continuous path of spots and causing magnetization inversion with the heat of the spot. Other proposed devices focus a laser beam on the medium to locally heat the medium so as to decrease its coercive force and magnetize the medium with magnetic fields that vary in accordance with signals to be recorded.
The latter means involve locally heating a medium by the energy of a laser beam so as to decrease the coercive force at a path of spots on the medium and to thermomagnetically write data with a magnetic field that acts on the spots.
Practically, however, if the laser light beam irradiates the medium, temperature distribution of the medium varies from a center 0 of the spot encountered by the laser beam as shown in FIG. 1. This means that the possibility exists of producing magnetization over a comparatively wide range around the center 0 of the spot. As a result, the magnetized region may be quite wide and this wide region may considerably reduce the available recording density.
Consideration has been given to applying an external magnetic field for preventing magnetization inversion caused by irradiation of the medium by a laser light beam, so as to limit the range of magnetization inversion and to prevent extension of the area of magnetization inversion at any one point.
However, as a practical matter, it has been very difficult to control and limit the magnetic field to produce a fine range of magnetization inversion.